The invention concerns a system for the generation of magnetic gradient fields which intermittently overlap the static magnetic field of a nuclear-spin tomograph, with at least one pair of gradient coils, powered from a power supply, through which current flows in series when in operation and which generate, in a predefined volume under examination, a gradient magnetic field that varies at least approximately in linear fashion with position in one direction, with each gradient coil having one set with at least one first winding zone and one set with at least one second winding zone, that have terminals separate from one another. In a system of this kind known from DE-A1-39 07 141, in order to reduce the electrical load on semiconductor elements, each winding zone is connected to a separate unit driver to deliver the current necessary for gradient generation. The, for example, two winding zones of a gradient coil can be operated in such a way that currents flow through them in the same direction with each gradient coil having one set with at least one first winding zone and one set with at least one second winding zone, that have terminals separate from one another. In a system of this kind known from DE-A1 39 07 141, in order to reduce the electrical load on semiconductor elements, each winding zone is connected to a separate unit driver to deliver the current necessary for gradient generation. Thus, for example, two winding zones of a gradient coil can be operated in such a way that currents flow through them in the same direction, or so that a current flows through only a single winding zone.
The invention also concerns a nuclear-spin tomograph with a gradient-generation system of this kind, and a process for the generation of images with a nuclear-spin tomograph of this kind.
Nuclear-spin tomographs, their operation, and possible imaging processes are exhaustively described in the technical literature (e.g. "Medizinische Physik," 1983, Huthig-Verlag, J. Schutz, ed.). In addition to a static magnetic field B.sub.o that is as homogeneous as possible, magnetic field gradients are also applied intermittently to an object being examined. A gradient field is a magnetic field which has the same direction as Bo, but whose strength varies linearly with position in one direction, and is constant perpendicular thereto. In general, a total of three different gradient fields are used, generated by a gradient-generation system with a plurality of gradient coils, with the directions of the corresponding gradients being perpendicular to one another (Gz=dBz/dz; Gx=dBz/dx; Gy=dBz/dy), where the z direction coincides with the direction of B.sub.o.
The linear change in field strength (constant gradient profile) should exist, if possible, throughout the object being examined. This places certain demands on the gradient coils with which the gradient fields are generally generated. Such coils are depicted and described, for example, in the article by J. Heinzerling on page 59 of the aforesaid work. In tomographs with a solenoid-type main field coil (for B.sub.o), the Gz gradient is usually generated with a so-called "anti-Helmholtz" arrangement, and the Gx and Gy gradients are generated with arrangements having four saddle coils each. Gradient coil sets adapted to the particular geometry are known for other magnet types, for example the "H magnet" (cf. DE 36 16 078), the "window frame" (cf. EP 181 383), or the type described in EP 167 639. Most of these coil arrangements are well known in high-resolution NMR, where such coils have been used for some time as static field-correction coils, or "shim coils."
When cylindrical coils are used as the gradient coils, in the simplest case only two coils--constituting the pair mentioned earlier--are needed in order to generate the gradient. As just mentioned, when saddle coils are used, four saddle coils are required for one gradient, each two of which constitute a single coil of the pair in the phraseology of the invention, so that the four saddle coils thus constitute the said pair.
Depending on the configuration of the gradient coils, the true gradient profiles correspond to the desired (constant) gradient profile over only a limited spatial region, which is often smaller than the object under investigation. Efforts have therefore been made to enlarge this region. This must naturally occur within certain existing constraints. With a solenoid-type main coil, the windings of the gradient coils are restricted to a cylindrical enveloping surface. Proposed solutions that essentially adopt the features known for shim coil sets are described, for example, in European patents EP-B-73399 and 73402. These solutions, which are capable of bringing higher orders of a gradient field development to zero, require either more coils or more turns than the single-coil solution. In addition, although in most cases the gradient field achieved with the use of higher-order coils is more homogeneous, it is also smaller than with single coils, assuming that the same energizing current is used in each case.
Most imaging processes in nuclear-spin tomography work with high-frequency pulse sequences, with the gradient fields also being switched on and off in a pulsed manner. The switching processes must therefore be as short as possible, but still defined and reproducible. The ideal case is a rectangular time profile or at least an approximately trapezoidal profile with the steepest possible edge (in the millisecond range). For a given inductance of the coil arrangement, a steeper leading edge requires a more powerful gradient power supplier to supply current to the gradient coils. It is therefore desirable to keep the inductance as low as possible, which is inconsistent with the requirement for a larger linearity region.
Pulsed gradient fields with rise times in the vicinity of a millisecond induce eddy currents in the surrounding conductive structure, which in turn lead to field distortions in time and space. One possible way of counteracting this problem is to use "self-shielding" gradient-generation systems, in which the gradient coils are surrounded externally by similar coils through which a current is sent in the opposite direction. This arrangement is configured so that the total magnetic fields largely compensate for one another on the outside, while a gradient field (which is as constant as possible) remains on the inside. Since more coils are available, both gradient generation and the shielding function can be optimized. Disadvantages include a considerably reduced gradient strength for the same energizing current. Self-shielding gradient-generation systems are described, for example, in EP-A-231 879, U.S. Pat. No. 4 733 189, and DE-A-3 808 995.
Since the known gradient-generation systems must satisfy contradictory requirements (maximum gradient strength, large homogeneity and linearity region, short rise times, no eddy current induction, etc.), it is fundamentally impossible to optimize them in every important respect. Known nuclear-spin tomographs have a single gradient-generation system (which comprises the aforesaid three gradient directions) that is either designed so that it represents a compromise for as many criteria as possible (universal system), or is optimized for an operator's particular needs. If those needs should change, they can be accommodated only by replacing the gradient-generation system. In particular, it is not possible to change the characteristics of the gradient-generation system during a single measurement (recording of a tomography image) or during a single measurement series.
Although EP-A-156 442 mentions operating a kind of combination shim and gradient-generation system in such a way that "the desired magnetic field profile in space and time" is achieved, this is to be understood to mean that a plurality of complex coil systems are activated by means of independent power supplies. The desired profile in space and time often requires currents that the power supplies cannot deliver, or tolerances that cannot be maintained. In addition, the configuration is very complex and expensive.